1. Field of the Invention
The present invention relates to apparatuses for optically reproducing information from and recording information onto recording media.
2. Description of the Related Art
To date, optical pickups having semiconductor lasers as light sources have been used in apparatuses for optically reproducing and recording information from and onto recording media.
Recently, wavelengths of the lasers have been decreasing as recording of information has been becoming more highly densified, and systems using blue-violet semiconductor lasers having a wavelength of 400 nm have been put to practical use.
Blue lasers are noisy at low powers of 3 mW or lower, and sufficient signal-to-noise (S/N) ratios of regenerative signals are difficult to achieve.
To solve this problem, methods for reducing laser noise by disposing neutral-density (ND) filters on optical paths during reproducing are proposed in Japanese Patent Laid-Open Nos. 6-13683 and 2002-150601, for example.
The noise of the semiconductor lasers is reduced as the output of the lasers is increased. Accordingly, the laser noise can be reduced by reducing optical efficiency during reproducing of the signals and by setting the output of the lasers to a high value.
In general, the shape of light beams emitted from the semiconductor lasers is elliptical.
When lasers having a large aspect ratio of the major axis of the ellipse to the minor axis are used, required resolution cannot be achieved either in a direction parallel to tracks or in a direction perpendicular to the tracks, resulting in unstable reproducing and recording.
Therefore, the sections of the light beams are approximated to isotropic circles with anamorphic prisms (so-called beam shaping).
FIG. 17 illustrates a known apparatus for optically reproducing and recording information.
Divergent elliptical light beams emitted from a semiconductor laser 101 are collimated by collimating lenses 102, and enter anamorphic prisms 104.
The light beams shaped into an approximate circle by the anamorphic prisms 104 pass through a polarized-beam splitter 105 disposed downstream of the anamorphic prisms 104.
Part of the incident beams is reflected, and is converged on a front-monitoring photodiode 107 by a monitoring lens 106.
The output of the semiconductor laser 101 is controlled on the basis of the output of this front-monitoring photodiode 107.
On the other hand, the light beams passing through the polarized-beam splitter 105 are converted into circularly polarized light beams by a quarter-wave plate 108, and the resultant beams enter a beam expander 109.
The beam expander 109 includes a combination of a concave lens and a convex lens. The concave lens is movable in the optical-axis direction so as to correct spherical aberration generated by an error in thickness of a protective layer on the recording medium.
The light beams passing through the beam expander 109 are converged on a recording medium 111 by an objective lens 110 having a numerical aperture (NA) of 0.85.
The light beams reflected from the recording medium 111 pass through the objective lens 110, the beam expander 109, and the quarter-wave plate 108, and are reflected by the polarized-beam splitter 105 so as to be guided to a photodetector 114 by a converging lens 112 and a sensor lens 113.
During reproducing of signals, an ND filter 103 is inserted into the optical path such that the output of the semiconductor laser 101 is set to high for noise reduction.
During recording of signals, the ND filter 103 is removed from the optical path.
In this manner, an optical pickup having high optical resolution and which is not easily influenced by laser noise can be achieved with the beam-shaping element and the light-attenuating element.
However, an apparatus including both the light-attenuating element and the beam-shaping element becomes large and results in an increase in cost.